Well, as it turned out, with the help of the info that Trev sent me, it was actually staggeringly simple to realign. Of course, nothing's that straightforward, is it ? After the tray was refitted and moving correctly when hand cranked from underneath via the little gear that they kindly give you to do just this, I put it back on power. The laser homed, the display said "TRAY INIT", and that's as far as it went, before spitting the tray out, coming to a stop, and the display changing to "TRAY OPEN". Nothing then worked at all, until it was powered back off. Looking at the area of the main PCB where the SMPS section is, there were several areas that were quite badly scorched, and numerous small(ish) electrolytics were located in these areas. A quick run over them with an ESR meter revealed four of them to be virtually open, so I figured that we may have dirty rails here, that were causing the micro to screw up during initialisation of the mechanics. Once the board was out, it was easy to see why it was discoloured. Panasonic, in a breathtakingly stupid bit of design work, have sited a whole bunch of sm regulator devices on the underside of this SRBP board, using the copper as a heatsink as well as an anchoring point. And then sited all the device decoupling caps immediately above. I mean, who designs this stuff in the first place, and then, what on earth is their manager thinking of, when he / she signs it off as an approved design ?
Anyway, I replaced the caps and reassembled. Result ? Nothing, of course. Exactly the same ... :-/
It then occurred to me that at no time had I ever seen the carousel rotating, so I removed the tray again - confident now that I could put it back in a flash - to have a look underneath at the drive motor and sensors. This little bit of circuitry is connected back to the main PCB underneath the deck, via a white flexiprint - the type with the blue strengtheners at either end. When I looked a bit closer, at the board end, it was very sharply bent at the inner edge of the strengthener. A quick ohms check revealed that several of the ribbons were fractured at this point of bend. I had a suitable length replacement for it in the junkbox, but it still didn't quite end there. The original was one of those rather more rare types, where the exposed connection 'fingers' at either end, were on opposite sides of the ribbon, rather than both on the same side, as is more common, and was the way my proposed replacement was. Fortunately, it was long enough to be able to cut off the fingers at one end, and then scrape back on the opposite side to reveal a new set. The strengthener was then glued back to the ribbon, and the whole shebang refitted.
This time, everything cycled correctly at power up, and each of the five disc positions was checked for the presence of a disc, before it all came to rest showing "NO DISC" in the display. It then opened ok, closed ok, and played a disc as it should.
Nice to get to the bottom of one that looked like it might be a non starter at first :-)
Arfa